[46.05] The organic volatile composition of six Oort-cloud comets: evidence for compositional diversity in the giant-planets' nebular region.

We investigated organic volatiles in six Oort-cloud comets
at infrared wavelengths, using CSHELL at NASA's IRTF and
NIRSPEC at the Keck observatory. For reasonably bright
comets, the production rates of seven parent volatile
species are measured in a space of two hours, with NIRSPEC.
These include H2O, CO, CH3OH, CH4, C2H2, C2H6, and HCN. We
extract intensities for multiple lines of select species,
permitting determination of rotational temperatures,
apparent production rates, and spatial profiles for these at
various points along the slit. (Spatial profiles are used to
eliminate systematic errors that would otherwise be
introduced by factors such as atmospheric seeing, and to
identify and compensate for optical depth effects. They also
permit measurement of asymmetric release from the nucleus,
and separate identification of native and distributed
sources for a given species.) Systematic errors are reduced
further when all species are measured with a single
instrument on a single telescope, and over a time interval
short compared with nucleus rotation. The resulting parent
volatile production rates are highly robust, permitting a
search for compositional diversity among these comets. Four
have similar chemistry (excepting CO) and independent
evidence reveals low formation temperatures (~ 30K), so
they probably originated beyond 30 AU. The fifth is depleted
in hypervolatiles and also in methanol, and likely formed
near 5 - 10 AU. The sixth is enriched in ethane - its
formation zone will be discussed. These results will be
discussed in the context of current models for interstellar
and nebular processing, and for delivery of volatiles to
Earth.

This work was supported by the NASA Planetary Astronomy
program (RTOP 344-32-30-07; NAG5-7905), the NASA Planetary
Atmospheres Program (NAG5-7753), and the NSF (AST-9619461).